1. Field of the Invention
The present invention relates most generally to treated gold or gold alloys and to copper, silver, palladium, aluminum and their alloys all of which becomes increasingly brittle with the accumulation of, or the integration of, time and temperature. More particularly, the present invention is directed to providing a means for the time temperature fusing of, or the protecting of, electrical or electronic systems by causing the mechanical or electrical rupture (resistivity increase) of a connecting wire of, for example, gold due to the increasing, with time and temperature, of the brittleness of the wire until such time as the resistivity increases to a predetermined value or until the wire breaks at a stress point in the wire.
2. Description of the Prior Art
There has been, and currently exists, a need to provide for the protection of both electronic and electrical circuits. In many circumstances it is desirable to disable a circuit system after it has been exposed to a specified temperature for a specified period of time. In order to provide this type of protection, or fusing, a temperature sensor such as a probe, a means for measuring elapsed time, a means for providing the product of time and temperature must be provided. Then these products of time and temperature must be accumulated. Finally when the critical value, or a threshold, is reached the threshold must be detected and a circuit disconnect must be caused to take place.
With the development of large scale integration techniques and the use of large numbers of integrated circuits within an electronic system, if it is desired to provide time temperature fusing of each integrated circuit or a portion of the integrated circuits within the system, it would be necessary to incorporate, at considerable expense, the time temperature integrator fusing system that has just been described. Alternatively it may be possible where the numbers of circuits permit, to use one or several time temperature integrator fuse systems and time-scan or time-share over the circuits which need to be protected. This approach is not one which holds great value for many obvious reasons. The number of circuits that can be time-shared would have to be minimal in number so that the time lapse between the time when a first circuit was being tested or examined for time temperature exposure to the time this circuit is again being examined for time temperature exposure must be within a reasonable period of time. Further, the temperature and time for each circuit would all have to be substantially equal to each other. Quite simply, there presently exists no economically nor technically feasible way to provide for time temperature integration fusing of a plurality of, for example, integrated circuits.
Given that there is specific concern for the time temperature fusing or the deactivating of integrated circuits upon the accumulated exposure to time and temperature of the integrated circuits, consideration was given to using the gold bonding wires, which are used to connect the integrated circuit to pins. These pins may be used to mount the integrated circuit or collection of interconnected integrated circuits to the rest of the electronic system. If a bonding wire could be modified or altered in such a way that with the integration of time and temperature the wire would physically break or the resistivity of the wire could be sufficiently increased, then the circuit associated with that particular gold bonding wire could be made nonfunctional or nonoperative. Providing a means for promoting a chemical reaction with the gold wire appeared, at first pass, to be a possible approach to be used to cause a change in either the physical or electrical characteristics of the gold wire. The difficulties associated with such an approach were recognized very early in the process of developing such a technology. For example, it is known that cyanides and iodides will react with the gold to form gold salts but these solutions are very active and produce vapors which are harmful and also react much too rapidly with gold. In addition, the quantities of solutions needed are very large, approaching 20 times the volume of the wire.
It was recognized by applicants herein that since gold is relatively soft (as is copper, silver, palladium and aluminum) it may be useful to consider changing the gold wire or other soft metal wire from being soft to being brittle and thereby effect the resistivity and physical properties. It was desired that these alterations in the properties of the gold or other soft metal wire be a function of elapsed time and also of the temperature. It is well known that intermetallics are brittle by nature. If, for example, a gold wire could be treated in a proper manner and using appropriate materials so that an intermetallic compound with the gold would be formed, then with time and temperature the material which forms an intermetallic with gold would diffuse into the gold wire from the surface by either a homogeneous diffusion or by diffusion along the grain boundaries (the rate of diffusion being dependent upon the temperature and also the length of time that diffusion takes place as well as the metal or metal alloy deposited onto or contained within the wire) the gold wire would progressively become intermetallic across the entire cross section of the wire and consequently more brittle and would break at a stress point or at stress points rendering nonfunctional a device or a system which uses the wire as an essential element. Alternatively the resistivity of the wire would be increased to such an extent as to render the circuit connected to the integrated circuit (IC) pins by the gold wire nonfunctional.
It is virtually always the case, for any number of reasons, that gold is used to plate over other metals. Presently there is no useful purpose known for plating over gold with a metal which will embrittle the gold. In fact it is very unconventional to cover over, deposit, or plate over gold, silver or palladium for any known reason or for any known useful purpose. In the integrated circuit industry, where gold wires are preferred, it is considered essential that the gold bonding wires be as pure as is possible. In complete opposition to conventional teaching and wisdom, in the instant invention pure gold has deposited on it a metal or an alloy of metals which will, with time and temperature, homogeneously diffuse into the gold or diffuse by way of the grain boundaries into the gold, which may be in the form of a wire, and create intermetallics which progress across the cross section of the gold wire or which diffuse into the grain boundaries and create intermetallics within the grain boundaries. In either case the gold wire becomes embrittled--the degree of brittleness increasing with time and temperature until the gold wire either physically breaks/ruptures or the resistivity of the wire increases to a level at which the circuit fails to function properly.
With proper selection of the intermetallic producing metals and/or alloys of such metals, (the intermetallics being formed with the soft metal or a soft metal alloy wire, substrate, ribbon or other geometric form of the soft metal or soft metal alloy) the time temperature fuse can be tailored to fail physically or electrically upon reaching a particular threshold value of integrated time and temperature. For example, gallium will diffuse rapidly into gold, create intermetallics and cause the embrittlement of the wire at relatively low temperatures. On the other hand, aluminum diffuses very slowly into the gold and thus it takes a considerably longer period of time for the gold to become embrittled and to reach the point of embrittlement where either the resistance of the gold wire increases to a level such that the circuit fails to operate or the wire becomes so embrittled that it physically breaks at a stress point.
In summary the invention can be described most generally as being soft metal or a soft metal alloy having deposited thereon, or contained within, a metal or a metal alloy which, as a function of time and temperature, homogeneously diffuses into and creates an intermetallic with the soft metal or soft metal alloy and thus embrittling the soft metal or soft metal alloy. Alternative to or simultaneously with homogeneous diffusion, grain boundary diffusion may occur; that is, diffusion along the grain boundaries creating an intermetallic within the grain boundaries and thereby embrittling the soft metal or its alloy.
It is a primary object of the invention to provide a composition of matter comprising a soft metal such as gold and at least one metal such as lead, indium, mercury, gallium, tin, bismuth and aluminum and the alloys of these metals which will diffuse into the gold thereby causing the embrittlement of the gold.
Another primary object of the invention is to provide a composition of matter comprising a soft metal alloy such as a gold alloy having a predetermined resistivity and having deposited thereon a material or metal alloy which will diffuse into the gold alloy or diffuse into the alloy along grain boundaries of the alloy thereby embrittling the alloy the embrittlement being a function of diffusion time and diffusion temperature.
Another object of the invention is to provide a method for causing the function interruption or the disfunction of an electronic or electrical circuit configuration where the disfunction takes place upon the accumulation of, or the integration of, time and temperature.
Yet another object of the invention is to provide a method for causing circuit disfunction dependent upon time and temperature and wherein the circuit disfunction is caused by the mechanical breaking or by the increase in resistivity of a soft metal wire such as a gold or gold alloy interconnecting wire within the circuit system and which breaking or increase in resistivity is caused by the embrittlement of the gold interconnecting wire with time and temperature exposure.
A still further object of the invention is to provide a method for causing the time temperature dependence embrittlement of a soft metal or an alloy of the soft metal such as gold or gold alloy due to the homogeneous diffusion or the grain boundary diffusion of metals or metal alloys which form intermetallics with gold or which form intermetallics throughout grain boundaries of the gold or gold alloy.